The present invention relates to electrical circuits for power conversion, for example, between different voltages and/or between AC and DC power, and in particular to a multilevel architecture for power converters that can provide improved power conversion of high voltages.
Conventional power conversion, for example, converting between different levels of AC power, may employ a transformer having inductively linked coils that transform input AC voltage to output AC voltage according to the turns ratio of coils.
For many applications, and in particular applications related to renewable energy including windfarms, electric vehicles, and photovoltaic arrays, transformer systems are unacceptably bulky, expensive, and inflexible. Windfarms and photovoltaic arrays may need to transform between low-voltage AC or DC to medium voltage grid power (2 kV to 35 kV) while recharging systems for electric vehicles may need to reduce medium voltage grid power to lower AC or DC voltages (200-400 volts) as needed for battery charging.
The cost, weight and bulk of the transformer may be reduced through the use of solid-state switching devices. In a “dual active bridge” design, solid-state devices convert input AC power to DC and then synthesize a higher frequency AC waveform that may work with a smaller transformer. Output from the transformer is then reconverted by additional solid-state devices to DC and then to the desired level of AC.
Preferably, a transformer-less medium voltage power conversion system could be developed by directly converting input AC power to the desired AC or DC output using semiconductor devices. Wideband (WBG) devices such as silicon carbide (SiC) MOSFETs may provide sufficiently high-frequency operation for this application and may have sufficiently high breakdown voltages to allow them to operate with power from medium voltage grids by connecting multiple devices in series.
Rapid switching of high voltages by solid-state devices can create problems of generating electromagnetic interference and high dv/dt (voltage change rate) such as can be damaging to electrical insulation on motors and the like. Placing solid-state devices in series can require careful coordination of the switching times of the devices to ensure that an individual device does not instantaneously experience excess voltages beyond their rated breakdown voltage.